Evaluation of the effects of the third molar on distalization and the effects of attachments on distalization and expansion with clear aligners: Three-dimensional finite element study

Objective: This study aimed to evaluate the effects of attachment design on maxillary molar distalization and simultaneous expansion during distalization, and the influence of third molars on distalization in the clear aligner technique using the finite element method. Methods: Six models were created to evaluate three different attachment designs on the second molars. Model I: employed a vertical rectangular attachment; Model II: used a vertical rectangular attachment with the presence of the third molar; Model III: used a combined semi-elliptical attachment; and Model IV: featured an opposed semi-elliptical attachment with buccal and palatal components. Models I through IV focused on distalization. Models V and VI were created by adding expansions to Models III and IV. The displacement amounts, clear aligner deformations, and stress distributions were analyzed using Ansys 19.2. Results: The presence of a third molar reduced maximum total displacement by 17%. Models I and III demonstrated similar distal displacement and tipping, both of which were more pronounced than in Model IV. Model IV achieved the most parallel tooth movement, with the least distal and buccal tipping and minimal distopalatal rotation. In the comparison of Models V and VI, Model VI exhibited greater distal and buccal displacements and more tipping than Model V. Conclusions: The presence of a third molar reduced distalization efficiency and increased clear aligner deformation. For both movements, the attachment that provided the most parallel movement showed the least displacement. When selecting attachments, it is essential to consider factors such as tooth rotation, inclination, bone support, and root health.